Why is it essential to the function of the cuspid valves that the muscles attached to them be well developed?

Answer 1

The mitral valve (left sided atrioventricular valve) and the tricuspid valve (right sided atrioventricular valve) are made of two and three leaflets respectively. They are large compared to the aortic and pulmonic valve leaflets (or cusps).

When blood is flowing through them normally, they basically flap in the "breeze" of the blood flow. When the ventricles contract though, they flap immediately backward very much like a parachute. There are parachute strings (called chords, for chordae tendinae) that attach to papillary muscles (so named because they are finger-shaped)

If these muscles were damaged (as can happen in some types of heart attacks), then the valves might not be fully effective in preventing backflow (or regurgitation) during the ventricular contraction. This backflow could be minor or major, and if major and sudden, could result in a very unpleasant and drawn out death for the victim as they gasp for breath with totally ineffective heartbeats.

The muscles themselves contract at certain points during the cardiac cycle, helping the efficiency of the valves. There isn't much you can do to exercise them to get into better shape, though. For most people they do fine for a long time.